PIN diodes are commonly used as switching elements in transmission lines operating in the microwave region and beyond. A PIN diode consists of a p-type semiconductor region separated from an n-type region by an intrinsic or i-type region. Because of their very small size they are commonly packaged to provide a more convenient size for interconnection with the transmission line. For instance, a packaged PIN diode is placed in a waveguide with one side in electrical contact with the waveguide wall and the other side connected, through a bias filter, to a source of bias voltage to operate the switch. It is customary to cascade several PIN diodes so as to obtain greater isolation, in which case the diodes are spaced one quarter of a waveguide wavelength apart, thus achieving an additional 6 db of isolation. While the isolation, which corresponds to a forward biased diode, is thus maximized, the insertion loss suffered when the diode is reverse biased is very high. The single degree of freedom availabe by adjusting the bias filter is not sufficient to optimize both states of the diode.
A portion of the capacitance which results in high insertion loss may be compensated for by very careful choice of the diode package. However, this technique depends for its repeatability on the precise geometry of the diode package and the characteristics of the individual diode, so it is not a reliable technique for production of more than a few switches.
The problems in optimizing switch performance, i.e., obtaining high isolation and low insertion loss, become substantial if transmission lines operating at millimeter wavelengths are considered. Other applications, even at lower frequencies, require isolation comparable to prior art switches and relatively low insertion loss. Moreover, the adjustment necessary to obtain best performance from prior art switches is time consuming and the remaining capacitance does not allow good performance over a wide bandwidth.